1. Field of the Invention
The present invention relates to a zero voltage switching power conversion circuit applied for a cold cathode fluorescent lamp (CCFL), more particularly, the power conversion circuit is able to provide high efficiency power by using the zero voltage switching (ZVS) technique.
2. Description of Related Art
With reference to FIG. 8, a conventional power conversion circuit for driving a cold cathode fluorescent lamp (CCFL) is shown. Such a power conversion circuit architecture is also named as a Royer Inverter. With the variation in input voltage, the output voltage level varies accordingly to result in undesirable change in the brightness of the CCFL. In order to overcome this problem, a buck circuit (60) is integrated in the power conversion circuit to maintain a stable output voltage. However, since it is difficult to increase the efficiency of the buck circuit (60), the power conversion efficiency of the whole circuit would also be implicated accordingly.
With reference to FIG. 9, another prior art of the power conversion circuit for driving the CCFL is briefly depicted with the block diagrams in which the pulse width modulation (PWM) technique is adopted. A high frequency ramp signal is compared with a feedback voltage in a comparator to generate a PWM signal. It is noted that a continuously supplied ramp signal is required and is unable to be temporarily shut down. When it is necessary to adjust the brightness of the CCFL, it is usually implemented by either directly modifying the pulse width of the PWM signal or comparing the PWM signal with a low frequency oscillation signal to derive control signals with On and Off periods. However, even during the “Off” periods, the power conversion circuit still continuously provides the high frequency ramp signal. Most of the time, the CCFL is operated in said brightness adjusting mode. Therefore, according to an equation P=CV2f, it is obvious to recognize that the circuit will meaninglessly consume a lot of power due to the continuously supplied high frequency ramp signal.
Therefore, it is desired to provide a novel power conversion circuit to obviate the aforementioned drawbacks.